A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes), which in turn vibrate the oval window and round window openings of the cochlea 104.
The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. FIG. 2 shows a cross-sectional view of the cochlea 104 in which the spiral shape is evident. The first full turn of the cochlea 104 is referred to as the basal turn, with the turns beyond that referred to as the apical turns. At each turn, the cochlea 104 has an upper duct, the scala vestibuli 201, and a lower duct, the scala tympani 202, which are separated by a middle duct, the scala media 203 that contains the sound sensing neural ends of the auditory nerve that lies along the center axis of the cochlea, referred to as the modiolar 204. The scala tympani 202 has an inner modiolar wall 205 and an outer lateral wall 206. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea. In such cases a cochlear implant is an auditory prosthesis which uses an implanted stimulation electrode to bypass the acoustic transducing mechanism of the ear and instead stimulate auditory nerve tissue directly with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processing stage 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant stimulator 108. Besides extracting the audio information, the implant stimulator 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through connected wires 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104. For a variety of reasons, the electrode array 110 is usually implanted into the scala tympani 202.
The electrode array 110 contains multiple electrode wires embedded in a soft silicone body referred to as the electrode carrier. The electrode array 110 needs to be mechanically robust, and yet flexible and of small size to be inserted into the cochlea 104. The material of the electrode array 110 also needs to be soft in order to minimize trauma to neural structures of the cochlea 104. But an electrode array 110 that is too floppy tends to buckle too easily so that the electrode array 110 cannot be inserted into the cochlea 104 up to the desired insertion depth. A trade-off needs to be made between a certain stiffness of the electrode array 110 which allows insertion into the cochlea 104 up to the desired insertion depth without the array buckling, and certain flexibility of the electrode array 110 which keeps mechanical forces on the internal structures of the cochlea 104 low enough.
Recent developments in electrode array designs and surgical techniques are directed towards minimizing the trauma of the surgical implantation of the array. For preservation of residual hearing it is of particular importance to preserve the natural intra-cochlear structures. Therefore, the size and mechanical characteristics of the electrode array are critical parameters for the best patient benefit. Some electrode array designs are pre-curved, though a drawback of that approach is that a special electrode insertion tool is needed which keeps the electrode array straight until the point of insertion.
As documented by Erixon et al., Variational Anatomy of the Human Cochlea: Implications for Cochlear Implantation, Otology & Neurotology, 2008 (incorporated herein by reference), the size, shape, and curvature of the cochlea varies greatly between individuals, meaning that an electrode array must match a wide range of scala tympani geometries. Furthermore, recently published research by Verbist et al., Anatomic Considerations of Cochlear Morphology and Its Implications for Insertion Trauma in Cochlear Implant Surgery, Otology & Neurotology, 2009 (incorporated herein by reference) has shown that the human scala tympani does not incline towards the helicotrema at a constant rate, but rather there are several sections along the scala tympani where the slope changes, sometimes even becoming negative (i.e. downwards). The location and grade of these changes in inclination were also found to be different from individual to individual. Consequently, electrode arrays should be highly flexible in all directions in order to adapt to individual variations in curvature and changes in inclination of the scala tympani for minimal trauma implantation.
FIGS. 3A-3B are x-ray photographs depicting the relationship between an implanted electrode array 301 and the side walls of the scala tympani. The electrode array 301 typically gets pushed outward during implantation to lie against the outer lateral wall 302 of the scala tympani. As can be seen in FIG. 3A, the cross-sectional size of the scala tympani is great enough compared to the size of the electrode array 301 so that the outer lateral wall location of the array is relatively far from the inner modiolar wall 303 (especially within the first basal turn). In some cases, as shown in FIG. 3B, the angle of the array entry into the cochlea brings the electrode array 301 closer to the modiolar wall 303 near the entry point, but that only lasts for a very short section before the distance again increases.
Electrode arrays that lie close to the inner modiolar wall of the cochlear scala tympani are advantageous over the more typical free-fitting electrode arrays that lie against the outer lateral wall in-terms of power consumption and effectiveness in stimulating the spiral ganglion cells of the modiolus. On the other hand, modiolar hugging electrode arrays create greater trauma during insertion (especially via a cochleostomy opening) and also during explantation.
Modiolar hugging electrode arrays known in the prior art are often pre-curved and required a positioning stylet for safe introduce it into the cochlea (e.g., U.S. Pat. No. 5,545,219, U.S. Pat. No. 6,125,302, and U.S. Pat. No. 6,374,143). Other existing perimodiolar hugging electrode arrays require some additional structural elements to ensure placement of the electrode array close to the inner modiolar wall after insertion. However, after insertion there is no opportunity for the surgeon to correct and optimize the position of the electrode array.
U.S. Pat. No. 6,498,954 describes a cochlear implant electrode array with a leading section that is attached to the distal end of the electrode array. Two separate cochleostomies are drilled, one at the base and another separate one at the apex of the cochlea. The electrode leading section then is inserted through the basal cochleostomy and advanced towards the apical cochleostomy. A forward end of the leading section is then pulled through the apical cochleostomy which causes the electrode array to be pulled into the cochlea. The leading section must be the leading section must relatively stiff in order to properly move the leading section through the interior of the cochlea from base to apex.